Horseradish peroxidase will be used to identify the location of the subnucleus of the medial rectus muscle of the cat. It is proposed to locate the cell bodies of those neurons that send axons to this subnucleus through the medial longitudinal fasiculus by means of iontophoresis of tritium labelled amino acid into the vicinity of the cell bodies of the paramedian reticular formation and the abducens nucleus. These cells will take up the amino acid, incorporate it into protein and distribute the protein throughout the length of the cell. Using photographic techniques, the course of the axons of these cells can be followed and those cells that project to the medial rectus subnucleus via the medial longitudinal fasciculus can be identified.